But the question of what types of events are considered legitimate Senate business is a central issue sure to be hashed out in the courtroom — one that extends into the wide-ranging audit currently underway in the upper chamber.
Duffy faces 31 charges, including fraud, breach of trust and bribery of a judicial officer. When police filed the charges in July 2014, they referred to "inappropriate expenses related to personal and partisan activity."
The RCMP went on to cite a series of dates where Duffy allegedly filed travel expense claims "containing false or misleading information," but did not provide more details.
A source familiar with the case told The Canadian Press that the Mounties approached the party during their investigation with questions about Duffy's activities on several dates.
The party dug into its files and called on riding association officials to cross-reference those dates with any fundraising events Duffy might have attended, providing spreadsheets to the RCMP. They also handed over details of expenses reimbursed to him, said the source, who was not authorized to disclose details and so spoke on condition of anonymity.
Duffy, a well-known former broadcaster, was a popular draw for Conservatives wanting to fill rooms with would-be supporters, including during the 2011 election campaign.
Two years ago, a Canadian Press report uncovered evidence suggesting Duffy had filed Senate expenses on some of the same days he was helping Conservative colleagues during the campaign.
A subsequent review by a Senate committee identified 49 days where per diems were allegedly claimed in Ottawa "during a time period ... that Senator Duffy does not appear to have been in Ottawa." After that review of dates in 2011-12, the file was forwarded to the Mounties.
The RCMP expanded the time frame, and included dates from 2009-12.
Duffy has maintained that some of the improper billing was due to administrative error.
More importantly, the Senate rules themselves are subject to different interpretations, and are also likely to be a focus in court. In describing the "principles of parliamentary life," they say, "Partisan activities are an inherent and essential part of the parliamentary functions of a senator."
The Senate travel policy of 2012 stated that "participation in party activities that are related to the work of the senator or the Senate and its proceedings" is "fully funded." Purely partisan matters such as election activities are not.
"We are confident that when the full story is told, as it will be, and shown to be supported by many forms of evidence, it will be clear that Sen. Duffy is innocent of any criminal wrongdoing," lawyer Donald Bayne said when the charges were laid.
The larger issue — whether Duffy's primary residence was in Ottawa or in Prince Edward Island — formed the heart of the controversy that began to unfold in late 2012. Duffy claimed expenses for his home in Ottawa as a secondary residence.
He reimbursed the Senate for $90,000 in expenses in the spring of 2013, with the help of a secret cheque from Nigel Wright, former chief of staff to Prime Minister Stephen Harper.
The senator has alleged the payment was part of a "monstrous fraud" cooked up by the Prime Minister's Office. Duffy said he was threatened with the loss of his Senate seat if he did not repay his expenses, which he did not believe he had claimed improperly. He said his living status had been cleared by senior members of the government.
Wright, who lost his job shortly after word of the cheque became public, always maintained he was acting in the public interest and that his only goal was to "secure the repayment of taxpayer funds." Last April, investigators announced they would not pursue criminal charges against him.
Before Duffy was turfed from the Conservative caucus in the spring of 2013, the Conservative party had paid $13,000 in Duffy's legal bills.
"Let me be clear: I violated no laws, I followed rules as they then were. And I never received a single note from Senate Finance or the leadership that suggested there was anything amiss," Duffy said in the fall of 2013.
In the end, Duffy was charged with bribery in connection with the $90,000 payment.
The trial is set to begin on April 7 and run until June.
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